Photolithography is often used in the manufacture of semiconductor devices and other electronic equipment. In photolithography, projection optics of high quality are often used to image features on a reticle onto a photosensitive substrate, such as a resist covered wafer. As the feature sizes desirable to be reproduced become ever smaller, the optical system or projection optics must be continually maintained and checked for image quality. Often, the performance of an optical system or projection optics is difficult to obtain without time consuming techniques. Generally, multiple exposures are required of a photosensitive substrate at different locations in the image field and at different focus depths to characterize the optical system. The optical system is then characterization by compiling information obtained from examining the multiple processed images. Each of the many exposures and the corresponding processed images are acquired serially. Consequently, focus errors, scan errors and temporal variations to the optical system parameters during the measurement are compounded. In the case of scan and focus errors, noise is introduced into the data. In the case of temporal variations, valid data are unrecoverable. Additionally, the data are discretely sampled rather than continuous across the parameter range. Consequently, quantization errors result from estimation of data values that lie between adjacent samples. With demand for increasing production throughput and increasing performance requirements of the projection optics capable of imaging reduced feature size, there is a need for improving the apparatus and methods used to characterize an optical system. There is a need to develop an apparatus and method that will quickly and easily provide high-precision data or information that can be used to characterize the performance of an optical system quickly and easily and with data obtained simultaneously and processed simultaneously without the need to perform multiple exposures and processing of multiple images.